Efficient OLED gets rid of heavy metals

The structure of the light-emitting molecule is key to the efficiency gains.

A new molecule increases the efficiency of fluorescent organic light emitting diodes without using heavy metals. Early OLEDs were made from fluorescent molecules that transform added electrical energy into light. However, rules of quantum mechanics make this transformation rather inefficient.

Here’s why. Pumping electricity through a fluorescent OLED excites charge carriers—electrons and positively charged “holes”—in the molecules. These charge carriers meet to form a bound state called an exciton. This exciton can be one of two types: a singlet exciton or a triplet exciton. That energy has to be released, but only singlet excitons can release it as light, and singlets only occur 25 percent of the time. The other 75 percent of excitons are triplets that relax by releasing heat, not light.

To increase the efficiency of these OLEDs, scientists add heavy metal atoms to help the previously heat-producing states emit light—with nearly 100 percent efficiency—through phosphorescence.

Now scientists have accomplished similar efficiencies without the metal. Hiroki Uoyama, of Kyushu University in Japan, and colleagues designed a molecule that has a small energy difference between the singlet and triplet excitons. That way, triplet excitons can cross over to an excited singlet state and then relax to emit light.

In this carefully designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

The molecule can be easily synthesized in one step, and its shape and structure are key to its increased efficiency. Tweaking the structure allows the researchers to build OLEDs that emit blue, green, yellow and orange light.

The lack of heavy atoms in the molecule means it might be cheaper to produce than phosphorescent OLEDs containing iridium or platinum, says Z. Valy Vardeny, at the University of Utah.

OLEDs are already finding their way into thin, high-def screens of some commercial devices. But building a molecule doesn’t necessarily mean it will find its way into device production, writes Brian D’Andrade of Exponent, Inc., in an accompanying commentary. To be useful in production, these molecules need to be optimized to emit colors of light needed in displays, they need to be able to be manufactured into devices, and the lifetime of those devices needs to be comparable—or better than—the state-of the art, he writes.

Basic research like this makes me excited. I realize that this tech might never find its way into consumer devices, but it is this kind of research that leads to the advances that do end up in consumer products.

Experimental OLEDS have been made that stayed over 400 cd/m^2 for longer than the lifetime of LEDs (which is reached at 50% their original brightness), but I believe what's in production still suffers from the main problem of short lifespans of blue, and the unequal degredation of brightness between colors.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

Hell yeah- go solid-state physics. I was talking with one of my conservative friends the other day who was utterly contemptuous of anything smacking of thin-film solar or OLED- said he: "Well that stuff will *never* see the light of day because it all relies on toxic metals." Just. You. Wait.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

For energy conversion from electricity to light that sounds pretty darn good. It's better than eg your average LED lights by a far margin - I believe those are around 15% tops, excepting some very special (and expensive) special cases. And often less.

Experimental OLEDS have been made that stayed over 400 cd/m^2 for longer than the lifetime of LEDs (which is reached at 50% their original brightness), but I believe what's in production still suffers from the main problem of short lifespans of blue, and the unequal degredation of brightness between colors.

Hmmm. The practicalities of mining the stuff and the destruction of ecology, the e-waste that occurs when the products are EOL'ed, the costs associated with refining the stuff, being beholden to foreign militaries and governments who control export etc. etc.

Phyllis Stein wrote:

Lead is only dangerous if you eat the stuff.

Chuck 1 million displays containing heavy-metals in a land-fill and watch as the water tables and local productive land gets polluted with high levels of nasties.

Basic research like this makes me excited. I realize that this tech might never find its way into consumer devices, but it is this kind of research that leads to the advances that do end up in consumer products.

If I recall correctly, Samsung was working on flexible screens as well and looking forward to put them in consumer products.

Experimental OLEDS have been made that stayed over 400 cd/m^2 for longer than the lifetime of LEDs (which is reached at 50% their original brightness), but I believe what's in production still suffers from the main problem of short lifespans of blue, and the unequal degredation of brightness between colors.

I was wondering about their longevity too, as the word "organic" hasn't been very popular in the digital age lately, with the organic dye in LTH Blu-ray discs causing them to start losing their data within months after being written.The article does not mention the longevity of this new molecule, if it degrades faster or slower than existing OLEDs. I suppose it is also not bright enough to be used as a light source, only for display panels.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

For energy conversion from electricity to light that sounds pretty darn good. It's better than eg your average LED lights by a far margin - I believe those are around 15% tops, excepting some very special (and expensive) special cases. And often less.

- Also it should be noted that an 'LED' screen is simply an LCD screen that is backlit by LEDs.

The white light generated by the LEDs passes through a polarising filter that absorbs around half the light. Reflective polarisers can let through more than 50% of the light as polarised light by reflecting and recycling the light that isn't of the selected polarisation.

After the polarising filter, the light passes through a colour filter, which for each of the three colours will block at least two thirds of the light.

Consequently, only about 20%, at most, of the generated light will make it out of the screen (when the screen is completely white). All of the light generated by an OLED is intended to be seen, so the efficiency depends only on the light generating efficiency and any transient absorption.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

For energy conversion from electricity to light that sounds pretty darn good. It's better than eg your average LED lights by a far margin - I believe those are around 15% tops, excepting some very special (and expensive) special cases. And often less.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

For energy conversion from electricity to light that sounds pretty darn good. It's better than eg your average LED lights by a far margin - I believe those are around 15% tops, excepting some very special (and expensive) special cases. And often less.

In this carefully-designed molecule, more than 90 percent of the excited states emit light. And it converts electrical energy to light with an efficiency of more than 19 percent, comparable to high-efficiency phosphorescent OLEDs.

That doesn't seem like a good efficiency rate at all. What's the current day efficiency rate of LED screens?

The point to take home is that 19% is way better than the ~1% incandescent bulbs have and there's no risk of heavy metal leaching into the environment when the device reahes end of life and goes into the waste stream.

BTW, 1 acoustic watt will burst your eardrums or kill you at close range. The overall efficiency of wideband acoustic transducers ("loudspeakers") is down around the 0.01% range